This invention relates to a variable valve timing engine including an exhaust gas recirculation device (hereinafter referred to as xe2x80x9cEGR devicexe2x80x9d) that recirculates part of exhaust gases back to a combustion chamber to reduce NOx emissions.
A conventional EGR device, for example, as disclosed in Japanese Laid-Open Patent Application, Publication No. 11-125126, A, includes an exhaust gas recirculation passage (hereinafter referred to as xe2x80x9cEGR passagexe2x80x9d) and an exhaust gas recirculation valve (hereinafter referred to as xe2x80x9cEGR valvexe2x80x9d). The EGR passage is provided to connect an intake passage and an exhaust passage with each other. The EGR valve is provided at a junction of an upstream end of the EGR passage and the exhaust passage to open and close the EGR passage. The EGR valve may open and close the EGR passage between a wide-open state and a tightly closed-off state inclusive to control a recirculation flow rate of exhaust gases flowing from the exhaust passage to the intake passage. The EGR valve opens, closes, or throttles based upon loads on the engine, engine speeds, temperature of cooling water, or the like, so as to recirculate exhaust gases at a controlled flow rate according to a varying driving state to the combustion chamber of the engine. A variable valve timing engine including a phase angle adjustment means that adjusts a phase angle between an exhaust cam for opening/closing an exhaust valve and an intake cam for opening/closing an intake valve is also known in the art.
Recirculation of part of exhaust gases through the EGR passage to the combustion chamber of the engine may advantageously restrict a steep increase in combustion speed and combustion heat, lowers combustion temperature, and thus reduces NOx emissions. Further, reduced pumping loss due to the recirculation may contribute to increased fuel efficiency.
However, as unburned fuel (hydrocarbon) in exhaust gases deposited on surfaces of the EGR passage and EGR valve gradually becomes carbonized with intense heat, the EGR passage or EGR valve is clogged with a growing blockage, which would disadvantageously interfere with recirculation of exhaust gases, producing unexpected extra emissions of NOx. Moreover, exfoliation of deposited carbon caused for some reason would change the rate of flow passing through the EGR passage, and disadvantageously produce extra emissions of NOx, as well. Further, change in overlap, or time during which the intake and exhaust valves are both open, through adjustment by the phase angle adjustment means would increase a flow rate of internal recirculation of the exhaust gases, and would make the EGR flow rate difficult to properly control according to the driving state, as the conventional EGR device includes no mechanism to control the flow rate of internal recirculation of the exhaust gases in accordance with the flow rate of recirculation of exhaust gases in the EGR passage.
Moreover, it is when the EGR valve is wide open to the maximum that the above conventional EGR device may recirculate the exhaust gases at the maximum flow rate. Therefore, if a required flow rate of recirculation of the exhaust gases corresponding to a driving state in the combustion chamber of the engine exceeds the maximum flow rate, the EGR device is disadvantageously unable to recirculate the exhaust gases in accordance with the required flow rate.
On the other hand, it is conceivable that a so-called internal EGR (internal exhaust gas recirculation) may be used to recirculate exhaust gases. The internal EGR utilizes a variable valve timing control or VTC, and simultaneously opens both of the intake valve and the exhaust valve in the engine, thereby recirculating part of the exhaust gases to the combustion chamber of the engine.
However, the variable valve timing control that determines timing of opening or closing the intake valve and the exhaust valve of the engine may preferably control power generation in the engine so that properties thereof may be adjusted to a best torque condition. This points to the limitations on the use of the variable valve timing control as a means of exhaust gas recirculation flow rate control; thus disadvantageously too much control by the variable valve timing control would preferably be avoided.
The present invention is created in view of the above-described circumstances, and it is an exemplified object of the present invention to provide a variable valve timing engine capable of detecting any clogging in the EGR passage to eliminate excess and deficiency of a flow rate of recirculation of exhaust gases, thus reducing NOx emissions. Another exemplified object of the present invention is to provide a variable valve timing engine that may recirculate exhaust gases in accordance with a required flow rate of recirculation of the exhaust gases even if the required flow rate is very large.
In order to achieve the above objects, there is provided a variable valve timing engine according to one exemplified aspect of the present invention comprising: an EGR passage through which exhaust gases are recirculated from an exhaust system to an intake system; a phase angle adjustment means that adjusts a phase angle between an exhaust cam for opening/closing an exhaust valve and an intake cam for opening/closing an intake valve; and a phase angle adjustment control means that controls the phase angle adjustment means in accordance with a flow rate of recirculation of the exhaust gases in the EGR passage.
If carbon once deposited in the EGR passage is exfoliated for some reason, a rate of flow passing through the EGR passage varies. However, the phase angle control means may actuate the intake valve and the exhaust valve to shift the phase angle therebetween in an advanced-angle or retarded-angle direction to increase or decrease the internal recirculation flow rate of the exhaust gases in accordance with the flow rate of recirculation of the exhaust gases in the EGR passage, so that a whole recirculation flow rate of the exhaust gases becomes a flow rate conformable to a driving state of the engine. Accordingly, the EGR flow rate control may be easily optimized in conformity with the driving state of the engine.
According to another aspect of the present invention, there is provided a variable valve timing engine comprising: an EGR passage through which exhaust gases are recirculated from an exhaust system to an intake system; a phase angle adjustment means that adjusts a phase angle between an exhaust cam for opening/closing an exhaust valve and an intake cam for opening/closing an intake valve, and adjusts overlap or time during which both intake and exhaust valves are open; and a phase angle adjustment control means that controls the phase angle adjustment means in accordance with a flow rate of recirculation of the exhaust gases in the EGR passage. In this embodiment, the phase angle adjustment control means restricts an phase angle adjustment amount by the phase angle adjustment means if the overlap based upon the flow rate of recirculation of the exhaust gases in the EGR passage is larger than the overlap based upon a driving state of the engine.
In the above constructions, since the phase angle between the intake cam and the exhaust cam may be adjusted so as to shift in an advanced-angle or retarded-angle direction in accordance with the flow rate of recirculation of the exhaust gases, the flow rate of recirculation of the exhaust gases as a whole becomes a flow rate conforming to a driving state of the engine, and thus the EGR flow rate control may be optimized easily in accordance with the driving state.
Further, if the overlap of the intake valve and the exhaust valve based upon the flow rate of recirculation of the exhaust gases in the EGR passage is larger than the overlap of the intake valve and the exhaust valve based upon a driving state of the engine, the above phase angle adjustment control means may restrict the phase angle adjustment amount, and the driving state of the engine may be prevented from getting worse.
According to yet another aspect of the present invention, the variable valve timing engine as configured as above may further comprise a clogging detection means that detects a clogging condition in the EGR passage, and the phase angle adjustment control means is configured to control the phase angle adjustment means in accordance with the clogging condition in the EGR passage.
As unburned fuel (hydrocarbon) in exhaust gases deposited on surfaces of the EGR passage and EGR valve were gradually becoming carbonized with intense heat, the EGR passage or an opening/closing portion of the EGR valve would be clogged, or blocked. However, the phase angle adjustment control means may actuate the phase angle adjustment means in response to the clogging, and adjust the flow rate of internal recirculation of the exhaust gases, so that the flow rate of recirculation of the exhaust gases as a whole may be a flow rate conforming to a driving state of the engine.
Further, according to yet another aspect of the present invention, the above clogging detection means may be configured to detect the clogging condition in the EGR passage during a fuel cutoff time.
In this construction, the clogging in the EGR passage may be accurately detected without making emission of the engine worse.
According to yet another aspect of the present invention, a variable valve timing engine as configured above further comprises: an EGR control means that controls an opening of an EGR control valve provided in the EGR passage; a valve timing adjustment means that adjusts opening/closing timing of the intake valve and the exhaust valve; and a valve timing control means that controls the valve timing adjustment means. In this aspect, the EGR control means and the valve timing control means are configured to control the EGR control valve and the valve timing adjustment means respectively in accordance with a required flow rate of recirculation of the exhaust gases.
According to the above embodiment, while the EGR is controlled in accordance with the requested flow rate of recirculation of the exhaust gases, the valve timing is controlled at the same time as appropriate. Therefore, even if the maximum flow rate provided when the EGR control valve is opened wide at the maximum is not more than the required flow rate of recirculation of the exhaust gases, the more flow rate of recirculation of the exhaust gases may be satisfactorily achieved as required with the internal EGR flow rate increased by the valve timing control.
Preferably, control by the EGR control means has a higher priority than control by the valve timing control means. In this embodiment, the flow rate of recirculation of the exhaust gases may be controlled without too much control by the valve timing control; thus, an influence on properties of power generation in the engine may be reduced.
As described above, it is not desirable to use the variable valve timing control as a means of exhaust gas recirculation flow rate control. In this respect, the valve timing control means may control the valve timing adjustment means after the EGR control means exercises control to make the opening of the EGR control valve to a maximum thereof. In this embodiment, the flow rate of recirculation of the exhaust gases is under control of the EGR control means to meet the required flow rate of recirculation until the EGR control valve is opened wide to the maximum, while when the EGR control valve is controlled to open to the maximum, and the flow rate goes beyond control of the EGR control means, the valve timing control means controls the flow rate of recirculation of the exhaust gases. Accordingly, the use of the valve timing control to achieve a desired flow rate of recirculation of the exhaust gases may be minimized, and thus the variable valve timing control may control power generation in the engine, for example, so that properties thereof may be adjusted to a best torque condition.
According to yet another aspect of the present invention, the above valve timing control means may control the valve timing adjustment means when the required flow rate of recirculation of the exhaust gases is within a non-sensitive range of the EGR control valve.
The EGR control valve used in the control of the exhaust gas recirculation cannot be open with a very small opening when the closed EGR control valve is going to open, thus requiring a moderately larger opening. Accordingly, if a required flow rate of recirculation of the exhaust gases is very small, the EGR control valve cannot be satisfactorily controlled. In contrast, the valve timing control means may control even a very small flow rate of recirculation of the exhaust gases. In this respect, the above embodiment is configured to allow the valve timing control means to have control of a very small flow rate within a non-sensitive range of the EGR control valve to satisfy a required flow rate with recirculation of the exhaust gases by the internal EGR. Consequently, even if a required flow rate of recirculation of the exhaust gases is very small, the exhaust gases may be recirculated at a flow rate satisfactory for the required flow rate to the combustion chamber of the engine.
Other objects and further features of the present invention will become readily apparent from the following description of preferred embodiments with reference to accompanying drawings.